The present invention relates to a method of forming a semiconductor device, and more particularly to a method of forming a shallow trench isolation with an isolation oxide film free of any divot and any void as well as a method of preventing formation of any divot and any void in the isolation oxide film of the shallow trench isolation.
The shallow trench isolation structure is used as an isolation structure in an advanced ultra large scale integrated circuit device for further improvements in scaling down and isolation property. This shallow trench isolation structure is superior in isolation characteristics as compared to a local oxidation of silicon structure. This shallow trench isolation structure is also smaller in occupied area than the local oxidation of silicon structure. Accordingly, the shallow trench isolation structure is more suitable for further increase in high integration of the integrated circuit. The number of the semiconductor device having the shallow trench isolation structure has been on the increase.
The conventional method of forming the shallow trench isolation structure will be described. FIGS. 1A through 1E are fragmentary cross sectional elevation views illustrative of semiconductor devices with shallow trench isolation structures in sequential steps involved in the conventional method of forming the same.
With reference to FIG. 1A, a thermal oxide film 4 as a pad oxide film is formed on a silicon substrate 2. A silicon nitride film 6 as a chemical mechanical polishing stopper is deposited on the thermal oxide film 4. A resist film is applied on the silicon nitride film 6. The resist film is then patterned by a lithograph technique to form a resist pattern 8.
With reference to FIG. 1B, the resist pattern 8 is used to carry out an isotropic etching to the silicon nitride film 6, the thermal oxide film 4 and the silicon substrate 2, whereby a trench is formed which penetrates the silicon nitride film 6 and the thermal oxide film 4 and reaches a predetermined depth from the surface of the silicon substrate 2.
With reference to FIG. 1C, the resist pattern 8 is removed. A plasma oxide film 10 is entirely deposited by a plasma enhanced chemical vapor deposition method over the silicon nitride film 6 and within the trench, whereby the trench is filled with the plasma oxide film 10. The plasma oxide film 10 is polished by use of the silicon nitride film 6 as a polishing stopper, whereby the plasma oxide film 10 remains only within the trench.
With reference to FIG. 1D, the silicon nitride film 6 is removed by a wet etching.
With reference to FIG. 1E, the pad oxide film 4 is further removed by a wet etching, whereby the isolation oxide film 10 projects upwardly from the surface of the silicon substrate 2.
The shallow trench isolation is superior in device characteristic but causes a problem in the fabrication processes. Divots are formed in the isolation oxide film. FIG. 2 is a fragmentary cross sectional elevation view illustrative of an isolation oxide film with divots. Namely, the silicon nitride film 6 as the chemical mechanical polishing stopper is removed, whereby the isolation oxide film 10 projects upwardly from the surface of the pad oxide film 4. This pad oxide film 4 is then removed by a wet etching. This wet etching is an isotropic etching. The pad oxide film 4 and the isolation oxide film 10 are also silicon oxide films. The wet etching is effective to the silicon oxide films, or not only the pad oxide film 4 but also the isolation oxide film 10. Namely, as the pad oxide film 4 is removed by he wet etching, the isolation oxide film 10 is also isotropically etched by the wet etching in isotropic directions as shown in arrow marks xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, and xe2x80x9ccxe2x80x9d thereby forming divots 22 in FIG. 2. After the removal of the pad oxide film 4, further wet etching processes for removing the silicon oxide films are carried out, whereby the divots are further enlarged.
A mechanism of forming the divots 22 will be described in detail. As described above, the wet etching is carried out to the pad oxide film 4. Since the wet etching is an isotropic etching. The etching direction is isotropic and includes the three directions xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d. The direction xe2x80x9caxe2x80x9d is a downward direction to etch the isolation oxide film 10. The direction xe2x80x9cbxe2x80x9d is a lateral direction to etch the isolation oxide film 10. The isolation oxide film 10 is etched in the two directions xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d before the pad oxide film 4 is completely etched. After the pad oxide film 4 has been completely etched, the isolation oxide film 10 is etched not only in the directions xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d but also in the direction xe2x80x9ccxe2x80x9d. The etching in the direction xe2x80x9ccxe2x80x9d forms the divots 22. Since the pad oxide film 4 is very thin, for example, has a thickness in the range of 5-30 nanometers, the pad oxide film 4 is removed in a very short time, whereby the surface of the silicon substrate 2 is shown and the etching to the isolation oxide film 10 in the direction xe2x80x9ccxe2x80x9d is started to form the divots 22.
After the wet etching process has been carried out and the divots 22 have been formed, a photo-lithography process is carried out. The divots 22 form a difference in level or steps which provides an influence of halation to the photo-lithography process. A dry etching is carried out to a film over the divots 22 by use of the resist mask prepared by the photo-lithography process, provided that the film has a variation in thickness due to the divots 22. This makes it difficult to ensure an accurate selectively and control to the shape.
The above conventional method causes a further problem as follows. As described above, the trench groove is filled by the CVD oxide film deposited by the plasma enhanced chemical vapor deposition method to form the isolation oxide fi within the trench. FIG. 3 is a fragmentary cross sectional elevation view illustrative of a trench isolation with a void formed in a trench in a semiconductor substrate in accordance with the conventional method. The isolation oxide film within the trench may have a void or a cavity 20. If the trench has a high aspect ratio of a depth to a width of the trench, a lateral direction deposition rate could not be ignored in relation to a vertical direction deposition rate, whereby the vertical direction deposition of the isolation oxide film by the plasma enhance chemical vapor deposition method is limited by the lateral direction deposition thereof. Namely, the deposition in the lateral direction could not be ignored, whereby the deposition rate in the shallow portion of the trench is made faster than the deposition rate in the deep portion of the trench. This makes it possible that the trench is made blockage in the shallow portion thereof, whereby the silicon oxide is no longer supplied to the deeper portion of the trench, whereby no longer deposition of the silicon oxide appears in the deeper portion of the trench. As a result, the void or cavity 20 is formed in the isolation oxide film in the trench. If the void or cavity 20 is formed in the isolation oxide film within the trench, a deep groove or depressed portion is formed in the isolation oxide film during the chemical and mechanical polishing method. This deep groove or depressed portion causes the same problem as described above. Namely, the deep groove or depressed portion form a difference in level or steps which provides an influence of halation to the photo-lithography process. A dry etching is carried out to a film over the deep groove or depressed portion by use of the resist mask prepared by the photo-lithography process, provided that the film has a variation in thickness due to the deep groove or depressed portion. This makes it difficult to ensure an accurate selectively and control to the shape.
In the above circumstances, it had been required to develop a novel method of forming a shallow trench isolation free from the above problem.
Accordingly, it is an object of the present invention to provide a novel method of forming a shallow trench isolation free from the above problems.
It is a further object of the present invention to provide a novel method of forming a shallow trench isolation to prevent formation of divots in an isolation oxide film for allowing a photo-lithography process and a subsequent dry etching process.
It is a still further object of the present invention to provide a novel method of forming a shallow trench isolation to prevent formation of void or cavity in an isolation oxide film for allowing a photo-lithography process and a subsequent dry etching process.
The present invention provides a method of forming a shallow trench isolation structure in a substrate. The method comprises the steps of: forming an isolation silicon oxide film which comprises an upper portion extending over a silicon oxide film over a silicon nitride film and a lower portion extending in a trench in a silicon substrate; and carrying out an isotropic etching to said upper portion of said isolation silicon oxide film and said silicon oxide film, thereby forming an isolation trench structure without divots in said trench in said silicon substrate.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.